Transmetallation between Metal-Only Lewis Pairs: A new rhodium alane complex

Article abstract of DOI:10.1039/c2cc35248f

In this communication, synthesis of a rhodium alane Lewis adduct, [Cp(Me₃P)₂Rh→AlCl₃], is reported. Given that direct synthesis using aluminium trichloride failed, a convenient transmetallation reaction was applied. The new MOLP was analysed by single crystal X-ray diffraction, multinuclear NMR spectroscopy and density functional theory calculations.

Full text of DOI:10.1039/c2cc35248f

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COMMUNICATION
Transmetallation between Metal-Only Lewis Pairs: a new rhodium
alane complexw
Jurgen Bauer, Holger Braunschweig* and Krzysztof Radacki
¨
Received 20th July 2012, Accepted 30th August 2012
DOI: 10.1039/c2cc35248f
In this communication, synthesis of a rhodium alane Lewis adduct,
[Cp(Me3P)2Rh-AlCl3], is reported. Given that direct synthesis
using aluminium trichloride failed, a convenient transmetallation
reaction was applied. The new MOLP was analysed by single
crystal X-ray diffraction, multinuclear NMR spectroscopy and
density functional theory calculations.
Calabrese investigated the reactivity of [Cp(R3P)2Rh] (5: R =
Me; 6: R = Et) towards aluminium acids.7 They identified
several adducts in solution with the acids AlMe3 and AlEt3,
however single crystals of these MOLPs could not be obtained.
The only crystallographically characterized product of their
investigations was the complex [Cp(Me3P)2Rh-Al2Me4Cl2]
(7a), which also can be regarded as a zwitterionic complex, i.e.
[Cp(Me3P)2RhAlMe2][AlMe2Cl2] (7b) with a weakly coordinated
counterion (Fig. 1).
In 1967, the first structural evidence for an unsupported, dative
metal–metal interaction was given by the X-ray molecular
structure of [Cp(OC)2Co-HgCl2] (1, Cp = Z5-C5H5, Fig. 1)
reported by Nowell and Russell.1,2 This was the first time that
an 18 valence electron (VE) complex was used as a Lewis base
to form an adduct between two metals. In the following years
related complexes of the complete group 9 were shown to also
yield ‘‘Metal-Only Lewis Pairs’’ (MOLPs).3 Examples are
the cationic bis-adduct [{Cp(OC)(Ph3P)Rh-}2Ag][PF6] (2)4
and the neutral adducts [Cp*(OC)2Ir-W(CO)5] (3, Cp* =
Z5-C5Me5)5 and [Cp(OC)2Rh-Pt(CO)(C6F5)2] (4).6
However, Dey and Werner further examined the reactivity of
5, for example in the synthesis of the adducts [Cp(Me3P)2Rh-
SnCl4] (8) and [Cp(Me3P)2Rh-HgCl2] (9).8 Although a GaCl3
adduct of a rhodium complex, [Cp*(Cp*Ga)2Rh-GaCl3]
(10, Fig. 1),9 is known, no fully characterized adducts between
a Lewis-basic rhodium complex and a simple trivalent aluminium
acid such as AlCl3 have been reported.
In contrast, with the elements of group 10, six AlCl3 containing
MOLPs are known for platinum centred bases10–12 as well as one
example for palladium.13 Our recent survey of MOLP complexes3
showed complexes of the form [CpRhL2] to be perhaps the
single most prominent transition metal Lewis base. Hence, we
became curious as to the absence of reports of analogous
rhodium AlCl3 adducts.
In addition, not only were MOLPs between transition
metals investigated but also the interaction of transition metal
bases with main group metal acids. In 1984, Mayer and
Complex 5 was prepared as reported by Werner et al.,14 and
subsequently reacted with AlCl3 analogous to the formation
of [(Cy3P)2Pt-AlCl3] (11) from [(Cy3P)2Pt] (12) and AlCl3
(see previous publications12 and the Experimental sectionz).
But in contrast to group 10 Lewis bases, no clean reaction
could be observed, even at low temperatures. Instead, the
31P{1H} NMR spectra revealed immediate formation of at
least seven products displaying a doublet due to the coupling
of the rhodium nucleus with the PMe3 ligands.
In order to better understand why the adduct formation was
unsuccessful, density functional theory (DFT) calculations were
performed, determining the bonding dissociation energy (BDE) of
the desired adduct [Cp(Me3P)2Rh-AlCl3] (13, Fig. 2). Surpris-
ingly, the BDE of 13 (ꢀ184 kJ molꢀ1) is about 40 kJ molꢀ1 more
negative than the BDE of [(Cy3P)2Pt-AlCl3] (11).15 In preliminary
investigations, we were able to confirm the theoretically-derived
BDEs of AlCl3 adducts with transition metal Lewis bases by
transfer experiments between platinum bases.10
Fig. 1 Selected published MOLPs with 18 VE half sandwich complexes.
Formally Lewis-basic fragments in blue, Lewis-acidic in red.
Institut fur Anorganische Chemie, Julius-Maximilians-Universitat
¨
¨
Wurzburg, Am Hubland, D-97074 Wurzburg, Germany.
¨
¨
E-mail: h.braunschweig@mail.uni-wuerzburg.de;
Fax: +49 931 31 84623; Tel: +49 931 31 85260
w Electronic supplementary information (ESI) available: General con-
siderations for experimental, computational, analytical and X-ray
diffraction analyses. CCDC 893198. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c2cc35248f
Therefore, we attempted to use the adduct 11 as a very mild
AlCl3 source in order to test a possible intermetallic transfer to
the bona fide stronger Lewis base 5 (Fig. 2).
c
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Fig. 2 Transfer of AlCl3 from 11 to the more Lewis-basic complex 5.
Indeed, after addition of 5 to a solution of 11 in C6D6,
immediate formation of the liberated platinum precursor 12
was detected in the 31P{1H} NMR spectrum. Furthermore, the
initially dark red colour disappeared after a few seconds with
formation of a pale yellow precipitate. After washing the solid,
which was obtained in sufficient yield (68% after work up),
and dissolving it in CD2Cl2, only one resonance, consisting of
a doublet at 1.0 ppm with a 1JRh–P coupling constant of 158 Hz,
was observed in the 31P{1H} NMR spectrum. Additionally, in
Fig. 3 Molecular structure of [Cp(Me3P)2Rh-AlCl3] (13). Relevant
bond lengths [A] and angles [1]: Rh–Al 2.425(1), Rh–P1 2.254(1),
Rh–P2 2.254(1), Al–Cl1 2.185(1), Al–Cl2 2.183(1), Al–Cl3 2.185(1),
P1–Rh–P2 97.2(1), P1–Rh–Al 91.9(1), P2–Rh–Al 90.3(1), Rh–Al–Cl1
113.3(1), Rh–Al–Cl2 110.4(1), Rh–Al–Cl1 120.6(1), Cl1–Al–Cl2
104.8(1), Cl1–Al–Cl3 101.6(1). Cl2–Al–Cl3 104.6(1). Ellipsoids drawn
at the 50% probability level; ellipsoids of the ligands and hydrogen
atoms were omitted for clarity.
1
the H NMR spectrum, only two resonances were found. The
resonance at 5.39 ppm was identified as the multiplet of a Cp
ligand and the other, at 1.65 ppm, shows the characteristic
3
splitting pattern of a doublet (due to the JRh–H coupling)
BDE and the energy needed for the steric rearrangement of the
fragments during adduct formation, is even more pronounced.
combined with the 2JP–H and 4JP–H coupling (Table 1). Thereby
Therefore, the actual metal–metal bond of 13 (ꢀ334 kJ molꢀ1
)
the formation of 13 was spectroscopically confirmed.
is about 53 kJ molꢀ1 stronger than the M–M bond of 11
(ꢀ280 kJ molꢀ1), which is reflected by the facile and rapid
transfer of the Lewis acid described before. Further evidence
for the enhanced Lewis-basic properties of 5 is the reduced
natural charge at the aluminium in 13 (AlCl3: 1.40; 11:
1.32; 13: 1.25; Table 2). Finally, the ADF program was applied
to determine the ratio of the electrostatic and covalent contribu-
tions to the overall interaction (see Table 2).15
Furthermore, the stability of 13 is extraordinary compared to
the reported platinum AlCl3 adducts. No signs of decomposi-
tion could be detected after storing 13 in dichloromethane for
more than a year while the platinum complex 11 decomposes
within one hour under the same conditions.
In order to determine the structure in the solid state, 13 was
recrystallized by slow diffusion of hexanes into a concentrated
solution of 13 in dichloromethane at ambient temperature.
The molecular structure of 13 (Fig. 3) displays the expected
geometry of a three-legged piano stool, with two molecules in
the asymmetric unit. Due to virtually identical parameters, only
one unit is discussed below. The bonding distance between
the two metals is (2.425(1) A) shorter than the distance in the
zwitterionic complex 7 (2.458(1) A, Fig. 1). The ratio of the
metal–metal distance to the sum of the covalent radii of
rhodium and aluminium (drel) is 0.922,16 and thus, smaller than
that of 7 (0.935). However, the drel values for the reported
platinum and palladium adducts with AlCl3 are all in the range
between 0.915 and 0.933.10–13
Additionally, calculation of the Kohn–Sham orbitals revealed
that the strongest interaction between the rhodium centred
orbitals with those of the Lewis acid is found in the HOMO.15
The main participating orbitals are a mixed orbital of rhodium
(ca. 40% dxz and contributions of 20% s and px, respectively)
and a mixed sp orbital of aluminium (40% s and px, respectively).
These findings are in good agreement with previous results
describing the donation of electrons from an occupied orbital
of the transition metal Lewis base to the vacant LUMO of the
Lewis acid AlCl3 (Fig. 4).
In summary, we report the first unambiguous alane adduct
of a group 9 transition metal, [Cp(Me3P)2Rh-AlCl3] (13),
In order to analyse the bonding situation in 13, further DFT
calculations were carried out.15 As mentioned before, the
rhodium–aluminium bond in 13 is, as judged from the BDE
(ꢀ184 kJ molꢀ1), stronger than the platinum–aluminium bond
in 11 (ꢀ141 kJ molꢀ1). Moreover, fragment analyses revealed
that the difference in the interaction energy, e.g. the sum of the
Table 2 Calculated geometric and electronic parameters of complexes 5,
11, 12 and 13
5
13
12
11
WBIa TMb–Al
Nat. charge TMc
Nat. charge Alc
BDEd
—
ꢀ0.14
0.38
ꢀ0.17
1.25
ꢀ184
37
—
ꢀ0.48
0.52
ꢀ0.31
1.32
ꢀ141
40
Table 1 Selected NMR spectroscopic parameters of complexes 5
and 13
—
—
e
Ereorg TMd
Ereorg Ald
113
99
e
5
13
f
31P{1H}a
ꢀ1.2
216
5.33
1.19
1
1.0
158
5.39
1.65
1
Einteraction
Eelstat
Eorbital
ꢀ334
52.3
52.3
ꢀ280
51.7
51.7
g
b
1JRh–P
g
1H (C5H5)a
1H (CH3)a
a
b
Wiberg Bond Index. Rhodium or platinum. Natural charge.
c
3JRh–H
b
d
f
e
. Reorganisation energy of the fragments.
Energies in kJ molꢀ1
|2JP–H + JP–H
|
8
10
4
b
g
Overall interaction. Percentage of the electrostatic and orbital
a
b
d in ppm. Coupling constants in Hz.
contributions.
c
This journal is The Royal Society of Chemistry 2012
10408 Chem. Commun., 2012, 48, 10407–10409

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solvent was decanted off. This procedure provided 11 in a yield of 92%
(108 mg, 0.121 mmol) as a yellow powder. 1H NMR (400.1 MHz,
C6D6): d = 2.64–1.10 (m, 66H); 31P{1H} NMR (162.0 MHz, C6D6):
1
d = 53.5 (d, JPt–P = 3017 Hz). For further analytical data see
ref. 12. [Cp(Me3P)2Rh(AlCl3)] (13): In a J. Young NMR tube,
[(Cy3P)2Pt(AlCl3)] (11) (28 mg, 31 mmol) was dissolved in benzene
and [Cp(Me3P)2Rh] (5) (10 mg, 31 mmol) was added. After a few
seconds the initial dark red colour disappears and a light yellow
precipitate is formed, indicating complete transfer of the alane fragment.
Then, the reaction mixture is filtered and the solid residue is dissolved in
dichloromethane. Layering this solution with toluene affords 13 as a
colourless solid (9.5 mg, 21 mmol, 68%). Crystals suitable for X-ray
diffraction were grown by slow diffusion of hexane into a dichloro-
methane solution of 13 at room temperature. 1H NMR (400.1 MHz,
4
CD2Cl2): d = 5.39 (m, 5H, C5H5), 1.65 (dvt, N = |2JP–H + JP–H| =
10 Hz, 3JRh–H = 1 Hz, CH3); 13C{1H} NMR (100.6 MHz, CD2Cl2): d =
92.4 (dt, 1JRh–C = 3 Hz, 2JP–C = 2 Hz, C5H5), 23.7 (dvt, 2JRh–C = 1 Hz,
N = |1JP–C
+
3JP–C| = 35 Hz, CH3); 31P{1H} NMR (162.0 MHz,
CD2Cl2): d = 1.0 (d, 1JRh–P = 158 Hz). Elemental analysis calcd. [%] for
C11H23AlCl3P2Rh: C, 29.13; H, 5.11%; found: C, 29.09; H, 5.13%.
Crystal data for 13: C11H23AlCl3P2Rh, Mr = 453.47, colourless block,
0.17 ꢁ 0.14 ꢁ 0.05 mm3, orthorhombic space group Pna21, a =
33.681(4) A, b = 12.6789(17) A, c = 8.5093(11) A, V = 3633.8(8) A3,
Z = 8, rcalcd = 1.658 g cm–3, m = 1.588 mm–1, F(000) = 1824, T =
100(2) K, R1 = 0.0187, wR2 = 0.0414, 7181 independent reflections
Fig. 4 Kohn–Sham orbital describing the interaction in the highest
occupied orbital (HOMO) of 13.
[2y r 52.741], 337 parameters and Flack parameter ꢀ0.009(15).
CCDC 893198.
1 I. N. Nowell and D. R. Russell, Chem. Commun., 1967, 817.
2 I. W. Nowell and D. R. Russell, J. Chem. Soc., Dalton Trans.,
1972, 2393.
3 J. Bauer, H. Braunschweig and R. D. Dewhurst, Chem. Rev., 2012,
8, 4329.
4 N. G. Connelly, A. R. Lucy, J. D. Payne, A. M. R. Galas and
W. E. Geiger, J. Chem. Soc., Dalton Trans., 1983, 1879.
5 F. W. B. Einstein, R. K. Pomeroy, P. Rushman and A. C. Willis,
Organometallics, 1985, 4, 250.
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8 K. Dey and H. Werner, J. Organomet. Chem., 1977, 137, C28.
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and its synthesis by transmetallation between two Lewis bases.
The approach herein presented shows significant potential for
the generation of MOLPs that have been to date inaccessible
by direct synthesis. Furthermore, computational predictions
of the enhanced Lewis basicity of the rhodium complex
[Cp(Me3P)2Rh] (5) could be affirmed by the clean and rapid
formation of 13 from the MOLP [(Cy3P)2Pt-AlCl3] (11) and 5.
Our results show that rhodium-centred Lewis bases may out-
perform corresponding platinum complexes not only in terms
of basicity, but also with respect to the stability of the obtained
Lewis pairs, even though the synthesis is more challenging. In
combination with the presented synthetic approach, this opens
up new fields in the ongoing research of MOLPs.
10 J. Bauer, H. Braunschweig, P. Brenner, K. Kraft, K. Radacki and
K. Schwab, Chem.–Eur. J., 2010, 16, 11985.
We are grateful to the German Science Foundation (DFG)
for financial support. J. Bauer is grateful to the Fonds der
Chemischen Industrie for a doctoral scholarship.
11 J. Bauer, R. Bertermann, H. Braunschweig, K. Gruss, F. Hupp
and T. Kramer, Inorg. Chem., 2012, 51, 5617.
12 H. Braunschweig, K. Gruss and K. Radacki, Angew. Chem., Int.
Ed., 2007, 46, 7782.
13 J. Bauer, H. Braunschweig, A. Damme, K. Gruss and K. Radacki,
Chem. Commun., 2011, 47, 12783.
Notes and references
14 H. Werner, R. Feser, V. Harder, W. Hofmann and H. Neukomm,
Inorg. Synth., 1989, 25, 158.
15 Details of the theoretical studies are provided in the ESIw.
16 B. Cordero, V. Gomez, A. E. Platero-Prats, M. Reves, J. Echeverria,
E. Cremades, F. Barragan and S. Alvarez, Dalton Trans., 2008,
2832.
z Experimental part: [(Cy3P)2Pt(AlCl3)] (11): [(Cy3P)2Pt] (12) (100 mg,
0.132 mmol) and AlCl3 (17.6 mg, 0.132 mmol) were dissolved in
benzene and stirred for 10 minutes at room temperature. After
irradiation in an ultrasonic bath for an additional 10 minutes the
reaction mixture was filtered and the solvent was removed in vacuo.
The residue was then suspended in 10 mL hexane, centrifuged and the
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 10407–10409 10409